The present invention relates to a method and apparatus for automatically changing a plating thickness in an electrical plating line. In a typical plating line of this general type, steel plates are plated with various metals. Specifically, the invention provides a plating current automatic switching device with which unsatisfactory plating before and after a plating thickness change point is minimized.
FIG. 1 is a block diagram of a conventional plating current switching device of the general type to which the invention pertains. In an actual installation, two such circuits are required for plating the two sides of a strip-shaped member. However, only one such circuit will be described as the two circuits are identical.
In FIG. 1, reference numeral 1 designates a plating current setting circuit which provides a total plating current set value for a plating thickness which is defined by the total capacity of available plating power sources and a line speed, 2 designates a speed proportion circuit for multiplying the total plating current set value by a proportional constant which varies in proportion to the line speed, 3 a proportional integrator circuit the output of which changes in such a manner that the output value from a total current feedback circuit 7 (described below) is made equal to the output of the speed proportion circuit, 4 current distributing circuits for distributing the output from the proportional integrator circuit 3 to the various plating tanks, 6 plating power sources for supplying currents to the plating tanks according to the output values from the current distributing circuits 4, 5 plating current switching circuits for selectively connecting and disconnecting the current distributing circuits to their respective plating power sources, and 7 a total current feedback circuit for detecting the total value of the plating currents and feeding the detected value back to the proportional integrator circuit 3.
The mechanical arrangement of the plating tank section is shown in FIG. 2. When a strip-shaped member to be plated passes through the plating tank section, the plating thickness is, in general, proportional to the sum of the currents of the plating power sources 6. Therefore, if the line speed is constant, the plating thickness is proportional to the plating current set value which is provided by the plating current setting circuit 1.
In a case where the plating thickness is to be changed at welding points on the member to be plated, the following conditions should be satisfied:
(a) The strip-shaped member should be suitably over-plated (overly thick plating) in certain limited areas because, if the plating thickness is insufficient before and after a plating thickness change point where the plating thickness changes, then the member will not be acceptable as a product.
(b) In order to prevent the occurrence of sparks in the gaps between the strip and conductive rolls 16 when a raised welding point passes through the conductive rolls 16, the plating power sources 6 before and after the welding point should be deactivated.
In the prior art approach, in order to satisfy the above-described two conditions, the line speed values to be employed before and after the plating thickness change point enters the plating line are compared. In the case where the line speed after the change is lower, when the change point arrives at the input side A of the plating tank section, the line speed and the plating current set value are changed to those which are required after the plating thickness change. In this case, the part of the strip-shaped member which is in the plating tank section and which is before the plating thickness change point (between the point A and a point B at the output side of the plating tank section) will be over-plated. On the other hand, in the case where the line speed after the plating thickness change point is higher, when the change point reaches the output side B of the plating tank section, the line speed and the plating current set value are changed to those which are required after the change. In this case, the part of the strip-shaped member which is in the plating tank section and which is after the plating thickness change point (between the points A and B) will be over-plated.
As is clear from the above description, in the conventional control method, a part of a strip-shaped member (before or after the plating thickness change point depending on whether the plating thickness is to be increased or decreased) is unavoidably over-plated because, when the plating thickness changes, the plating currents for all the plating tanks must be changed simultaneously. As a result, there is a large waste of plating material, which can be quite costly when expensive plating metals are being used.